1. Field of the Invention
The present invention relates generally to an optical device, terminal apparatus, and system for wavelength division multiplexing suitable for an increase in transmission capacity, and more particularly to an improvement in an optical device usable as an optical multiplexer and/or an optical demultiplexer.
2. Description of the Related Art
In recent years, the processing of enormous amounts of information has been needed with the evolution of an advanced information society, and optical fiber communication fit for a large capacity has been applied to a transmission network for transmitting information. While a transmission rate of information in optical fiber communication has already reached 2.4 Gb/s or 10 Gb/s, a further increase in transmission capacity will be needed in a motion picture captured communication system that is expected to be put to practical use in the future. For example, a transmission capacity of more than 1 terabit per second (Tb/s) will be needed in a trunk system.
Research and development on wavelength division multiplexing (WDM) is being actively carried out to increase a transmission capacity in optical fiber communication. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The optical carriers are individually modulated to obtain a plurality of optical signals. The optical signals are next wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light is next launched into an optical fiber transmission line. At a receiving end, the WDM signal light received is separated into a plurality of individual optical signals by an optical demultiplexer to reproduce transmitted data according to each optical signal. Accordingly, by applying WDM to such a system, a transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
As an optical device usable as the optical multiplexer and/or the optical demultiplexer, an arrayed waveguide grating (AWG) is known in the art. For example, the AWG includes a first slab waveguide to which a plurality of input optical waveguides are connected, a second slab waveguide to which a plurality of output optical waveguides are connected, and a plurality of connecting optical waveguides provided between the first and second slab waveguides and having different optical path lengths. The connecting optical waveguides and the first and second slab waveguides operate as a diffraction grating, so that each input optical waveguide and each output optical waveguide are coupled by a specific wavelength. Accordingly, by connecting the input optical waveguides to a plurality of optical transmitters, respectively, and connecting one of the output optical waveguides to a single optical fiber transmission line, this AWG functions as an optical multiplexer at a transmitting end. Further, by connecting one of the input optical waveguides to a single optical fiber transmission line and connecting a plurality of optical receivers to the output optical waveguides, respectively, this AWG functions as an optical demultiplexer at a receiving end.
Thus, an AWG may be provided by optical waveguides, thereby allowing size reduction of an optical multiplexer and an optical demultiplexer. However, the insertion loss by an AWG is generally large, so that in a system having an AWG as an optical multiplexer and/or an optical demultiplexer, there is a possibility of degradation in transmission quality. Further, in the case that the AWG is provided by optical waveguides, a highly skilled technique is required to connect the AWG to an optical fiber transmission line, so that it is not easy to manufacture an optical device usable as an optical multiplexer and/or an optical demultiplexer.
As a conventional more typical optical device usable as an optical multiplexer and/or an optical demultiplexer, there is an optical device including a plurality of optical filters each having a dielectric multilayer film or the like. For example, the optical device is configured by cascading a plurality of bandpass filters, so as to perform demultiplexing of WDM signal light. Each bandpass filter has a passband including a corresponding wavelength and excluding the other wavelengths. In this optical device, the insertion loss by each bandpass filter is accumulated from the upstream side toward the downstream side, so that there is a problem of variation in the insertion loss between the wavelengths.
To cope with this problem, it may be proposed to apply an optical filter having an input port and first and second output ports and functioning as a WDM coupler. This optical filter couples the input port to the first output port for a group of wavelengths longer than the cutoff wavelength of this filter, and couples the input port to the second output port for a group of wavelengths shorter than the cutoff wavelength. In other words, this optical filter is a long-wave pass filter or a short-wave pass filter. By applying the above-mentioned optical device having a plurality of bandpass filters to each of the group of longer wavelengths and the group of shorter wavelengths, the variation in insertion loss can be half reduced. However, in the case that the wavelength spacing is narrow as in dense WDM, a loss tilt (a tilt in wavelength characteristic of loss or a wavelength derivative of loss) near the cutoff wavelength must be set sufficiently large. As a result, the design of optical parameters of a dielectric multilayer film becomes complicated, for example, causing a problem that the manufacture of the optical device is difficult.
It is therefore an object of the present invention to provide an optical device usable as an optical multiplexer and/or an optical demultiplexer which can be easily manufactured and can reduce a maximum variation in insertion loss between wavelengths.
It is another object of the present invention to provide a terminal apparatus and a system each including such an optical device.
Other objects of the present invention will become apparent from the following description.
In accordance with an aspect of the present invention, there is provided an optical device comprising a WDM port adapted to wavelength division multiplexing (WDM); first to N-th ports to which first to N-th wavelengths are respectively allocated, where N is an integer greater than 4; and first to fourth optical filters. The first optical filter couples said WDM port to said i-th port by said i-th wavelength, where i is an integer satisfying 3xe2x89xa6ixe2x89xa6(Nxe2x88x922), and also couples said WDM port to said second optical filter by the plural wavelengths except said i-th wavelength. The second optical filter couples said first optical filter to said third optical filter by said first to (ixe2x88x921)-th wavelengths, and also couples said first optical filter to said fourth optical filter by said (i+1)-th to N-th wavelengths. The third optical filter couples said second optical filter to said first to (ixe2x88x921)-th ports respectively by said first to (ixe2x88x921)-th wavelengths. The fourth optical filter couples said second optical filter to said (i+1)-th to N-th ports respectively by said (i+1)-th to N-th wavelengths. The i-th wavelength may comprise a plurality of wavelengths.
In this optical device, the first optical filter separates WDM signal light into an optical signal having the i-th wavelength and optical signals having the other wavelengths. Accordingly, a long-wave pass filter or short-wave pass filter having a large loss tilt near the cutoff wavelength, for example, can be used as the second optical filter, so that this optical device can be easily manufactured. In this case, the cutoff wavelength is set substantially equal to the i-th wavelength (in the case that the i-th wavelength comprises a plurality of wavelengths, the cutoff wavelength is set substantially equal to a central one of these plural wavelengths). Furthermore, the second optical filter separates the remaining wavelengths into a group of wavelengths longer than the cutoff wavelength and a group of wavelengths shorter than the cutoff wavelength. Accordingly, a maximum variation in insertion loss between the wavelengths can be reduced.
In accordance with another aspect of the present invention, there is provided a system comprising first and second terminal apparatuses, and an optical fiber transmission line connecting said first and second terminal apparatuses. The first terminal apparatus comprises a plurality of optical transmitters for outputting a plurality of optical signals having different wavelengths, and an optical multiplexer for wavelength division multiplexing said plurality of optical signals and outputting resultant WDM signal light to said optical fiber transmission line. The second terminal apparatus comprises an optical demultiplexer for separating WDM signal light transmitted by said optical fiber transmission line into a plurality of optical signals having different wavelengths, and a plurality of optical receivers for receiving said plurality of optical signals output from said optical demultiplexer. At least one of said optical multiplexer and said optical demultiplexer comprises an optical device according to the present invention.
In accordance with a further aspect of the present invention, there is provided a terminal apparatus comprising a plurality of optical transmitters for outputting a plurality of optical signals having different wavelengths, and an optical multiplexer for wavelength division multiplexing said plurality of optical signals. The optical multiplexer comprises an optical device according to the present invention.
In accordance with a still further aspect of the present invention, there is provided a terminal apparatus comprising an optical demultiplexer for separating wavelength division multiplexed signal light into a plurality of optical signals having different wavelengths, and a plurality of optical receivers for receiving said plurality of optical signals. The optical demultiplexer comprises an optical device according to the present invention.
In the system or the terminal apparatus according to the present invention, an optical device whose maximum variation in insertion loss between the wavelengths is small is used as the optical multiplexer and/or the optical demultiplexer, so that a transmission quality can be improved.